Fibrinolysis: A Critical Determinant in Musculoskeletal Tissue Repair

Abstract

For over a century, the formation of a fibrin clot has been considered to be an initial and fundamental phase of musculoskeletal tissue repair. Following a severe injury, accumulation of fibrin clots is necessary to stop bleeding, yet recent work in our lab has demonstrated that proper healing of both bone and skeletal muscle is dependent on the temporal nature of fibrin and the fibrinolytic mechanisms rather than fibrins presence in damaged tissues.Fibrinolysis in Fracture HealingFibrin, the principle polymer protein of the coagulation system, was long believed essential to fracture healing by providing the initial scaffold for osteo‐regenerative mediators. Hence, many principles of musculoskeletal injury care and pharmaceuticals have been developed to provide a fibrin matrix in the fracture bed thought to represent the initial scaffold of fracture healing. However, we have recently demonstrated that fibrin is not essential for fracture healing, rather, persistent fibrin, which was not effectively removed via fibrinolytic mechanism, impaired fracture healing and inhibited union. Specifically, by creating a mid‐shaft femur fracture on 8 week old male mice (Wild type (WT)); n= 15, plasminogen deficient (Plg−/−) which cannot remove fibrin; n= 14, and fibrinogen deficient (Fbg−/−) which cannot make a fibrin clot; n=6) we revealed that WT and Fbg (−/−) mice formed ossified bridging callus by 3 weeks post fracture which was almost completely remodeled and all united by 6 weeks post fracture. Alternatively, Plg (−/−) mice failed to develop an organized mineralized callus and failed to unite or remodel the disorganized matrix (Figure 1A). Consistent with this finding, there was abundant avascular cartilage in Plg (−/−) mice at 2 weeks post fracture as compared to WT mice. Fibrin and CD31 immunohistochemical (IHC) staining showed abundant fibrin interposed between the avascular cartilage and vascularized bone (Figure 1B and C). Finally, reducing fibrinogen levels in Plg (−/−) mice with a targeted antisense oligonucleotide partially rescued the fracture healing in these mice. Considering that many conditions which result in pathologic fracture healing such as diabetes, smoking and aging all have impaired fibrinolysis resulting in fibrin accumulation in tissues, these results may provide valuable insight into novel means of improving fracture healing in these populations by targeting fibrin degradation.Fibrinolysis in Skeletal Muscle HealingLike bone, a fibrin meshwork forms in damaged skeletal muscle following injury, yet is resolved by the main protease of the fibrinolytic system, plasmin, to allow for timely and robust tissue regeneration. Through recent work in our lab, we have demonstrated that the protease plasmin, in addition to its canonical fibrinolytic function, also works to prevent the aberrant formation of mineralization within soft tissues. Specifically, extensive or persistent calcium phosphate deposition within soft tissues after a traumatic injury or major orthopedic surgery can result in pain and loss of joint function. The pathophysiology of soft tissue calcification, including dystrophic calcification and heterotopic ossification (HO), is poorly understood. Yet, we have demonstrated that after muscle injury, insufficient plasmin activity can lead to the formation of dystrophic calcification (Figure 2A). Additionally, we have found that if the dystrophic calcification is persistent within damaged tissues, it can become organized into mature bone, known as HO. Importantly, we determined that downregulating the primary inhibitor of plasmin (α2‐antiplasmin) pharmacologically prevented dystrophic calcification and subsequent HO in vivo (Figure 2A and Figure 2B). Since plasmin also supports bone homeostasis and fracture repair, increasing plasmin activity represents the first pharmacologic strategy to prevent soft tissue calcification without adversely affecting systemic bone physiology or concurrent muscle and bone regeneration.Together these investigations have shifted the paradigm of the role of fibrinolysis in musculoskeletal tissue repair. By utilizing animal models that effectively phenocopy different forms of human musculoskeletal injury, we aspire to translate these findings to the development of novel therapeutics aimed at improving tissue regeneration following a severe injury.Support or Funding InformationFunding for this work was provided by the Vanderbilt University Medical Center Department of Orthopaedics and Rehabilitation, The Caitlin Lovejoy Foundation, RO3 (1RO3AR065762), Orthopedic Pilot Program Award, The Musculoskeletal Transplant Fund, HHMI Student Fellowship, The Orthopaedic Trauma Association, Vascular Biology Training Grant‐Postdoctoral Funding, and the Predoctoral Pharmacology Training Grant (T32 GM007628).